Many metals and alloys are an important category of materials due to the various industrial applications and economic importance but they are susceptible to different mechanisms of corrosion due to their exposure to different corrosive media [1, 2, 3, 4]. The corrosion behaviour of these materials depends of the oxide layer on metal surface and a great number of corrosion investigations performed using different techniques: electrochemical [3,4], SERS [5,6], XANES [7,8] etc., have been focused on the surface film. Because of the general aggressiveness of chloride-containing solution [9, 10] corrosion control is an essential issue from application point of view. One of the important methods used to reduce the rate of metal corrosion is the addition of inhibitors [11, 12, 13, 14]. Corrosion inhibitors are substances, which in small concentrations decrease or prevent the degradation of metal. The degree of inhibition depends on metal nature and type of the medium.
Unfortunately, many of the inhibitors used are compounds with toxic properties. However, an increasing awareness of health and ecological risks has drawn attention to finding nontoxic and environmental friendly corrosion inhibitors. Therefore, researchers are looking for eco-friendly natural inhibitors as an alternative [12, 13, 14, 15, 16, 17]. Green inhibitors can be used in the form of extracts, essential oils or pure compounds. The natural compounds are biodegradable, easy available and non-toxic. Plants, which contain heterocyclic constituents (phenolic, aromatic compounds, etc.) can act as potential inhibitors. Protection efficiency of these inhibitors is attributed mainly to the presence of an active center for adsorption.
The possibility of corrosion inhibition of iron by natural antioxidants from Satureja montana L. was the focus of this study. In our earlier paper  the extract of the Achillea millefolium L. was investigated as corrosion inhibitor on a iron sample in an acid rain solution. The aim of the present work is to study the influence of Satureja montana L. extract on the corrosion behaviour of iron in NaCl solution.
Satureja montana L. is a plant with antimicrobial [19,20], cytotoxic , and antioxidant properties [22, 23, 24, 25] and it is also used in ethno medicine against intestinal parasites and for the treatment of stomach disorders and cough. According to scientific papers [23, 24, 25] this activity of SM extract is attributed to presented phenolic compounds. Due to positive properties, Satureja montana L., was chosen as a potential environmentally friendly corrosion inhibitor.
The study was performed with spectroscopically pure iron (Fe). The sample of Fe cylinder was used as a working electrode. The lateral surface of the cylinder was coated with polymer material, exposing only the plane of the cylinder base with a 0.636 cm2 surface area to the solution.
The surface of the electrode was polished with 1200 grade emery paper, degreased in ethanol in an ultrasonic bath and rinsed with ultrapure water produced by Millipore Simplicity UV Water Purification System.
A Pt plate and an Ag | AgCl | 3 mol L-1 KCl were used as the counter and reference electrodes, respectively. All potential values are reported vs. Ag | AgCl | 3 mol L-1 KCl. Measurements were carried out at room temperature in a standard three electrode arrangement using Autolab PGSTAT320N controlled by a personal computer using Nova 1.5 software.
The electrolyte was purified with argon. Prior to electrochemical measurements the electrode was held at the potential -1.40 V vs. Ag/AgCl for 30 seconds to remove spontaneously formed oxides.
Potentiodynamic polarization (PP) curves were recorded in the potential range from -400 mV to 400 mV near the open circuit potential (EOCP) and with the scan rate of 1.0 mV s-1.
Cyclic voltammetry (CV) measurements were performed in the potential region from –1.40 V up to –0.20 V with a sweep rate of 40 mV s-1.
Electrochemical impedance spectra (EIS) were performed at open circuit potential, E OCP in the frequency range 10 kHz – 5 mHz with a 10 mV rms amplitude. Prior to EIS measurements the electrode was immersed in the solution for 30 minutes to stabilize at the open circuit potential. All experiments were performed at least thrice to ensure the reproducibility of the data.
The content of dissolved iron during 1 hour-immersion of electrode in solution at EOCP were determined by atomic absorption spectrometry (AAS). This period of time was chosen to match the duration of EIS measurments.
The basic electrolyte was 0.1 mol L-1 sodium chloride solution. The pH value of solution was 7.0. pH value was adjusted by the addition of 0.2 mol L-1 NaOH and of 0.2 mol L-1 HCl. All chemicals used were of analytical grade and were obtained from Sigma Aldrich and all solutions were prepared with ultra pure water.
Experiments were performed in sodium chloride solution without and with the addition of Satureja montana L. extract (SM) and phenolic fraction of Satureja montana L. extract (PF). The aerial parts of Satureja montana L. were collected during summer 2017 in the wider area of Mostar, Bosnia and Herzegovina and specimens were dried in air for three weeks. Extract of plant material was prepared after 6 h maceration in water at room temperature. The concentration of the Satureja montana L. extract was 2.0 g L-1. 1 mL SM was added in 50 mL sodium chloride solution.
The neutral and acidic phenolic constituents of extract were separated using solid-phase extraction (SPE) on C18 Sep Pak cartridges (Waters Associates, Milford, MA, USA) according to the procedure outlined by Katalinić . Briefly, 10 mL of extract was passed drop by drop through cartridges, previously conditioned with methanol and water. The acidic phenol fraction was eluted with 10 mL of water and the neutral phenolic fraction (PF) was eluted with 10 mL of ethyl acetate, evaporated under vacuum on a rotary evaporator at 30oC and the dry residue dissolved in 10 mL water. In this work the neutral phenolic fractions were used for investigation, 1 mL PF was added in 50 mL sodium chloride solution.
The total phenolic compounds of Satureja montana L. were extracted by 70% aqueous solutions of acetone [27,28]. Extraction was performed using 0.5 g of plant material and 25 mL solvent during 20 min in an ultrasonic bath at room temperature. The slurry was filtered. Then, extract was centrifuged at 3000 rpm for 10 min. at 4oC. The supernatant was recovered and used for the determination of total phenolic contents.
Total phenolics of Satureja montana L. extract were determined spectrophotometrically using Folin-Ciocalteu reagent [27, 28, 29]. Different concentrations of tannic acid (20 - 120 μg mL–1) were used to plot a standard curve (y = 0.0064 x + 0.0016; r2 = 0.9952). Aliquot (100 μL) of aqueous acetone extract was transferred into the test tube and was diluted with water to the final volume of 0.5 mL. To each tube, 0.25 mL of the Folin-Ciocalteu reagent and 1.25 mL of the Na2CO3 solution (20%) were added. After 40 min. at room temperature, the absorbance of all samples and standards was measured at 725 nm using a UV-Vis spectrophotometer (Shimadzu, 1240). The total phenolic contents were calculated according to the tannic acid and they were expressed as tannic acid equivalents per g dry sample.
Ethical approval: The conducted research is not related to either human or animal use.
3 Results and Discussion
3.1 Potentiodynamic polarization
Figure 1 shows the potentiodynamic polarization curves for the Fe electrode in 0.1 mol L-1 NaCl in the absence and presence of extract (SM) and phenolic fraction (PF) of Satureja montana L.
It is obvious from Figure 1 that the presence of SM and PF induces changes in the polarization behaviour of the iron and shifts cathodic curves to lower values of current densities compared with those in the pure 0.1 mol L-1 NaCl solution. The addition of both extracts shifts the corrosion potential to more negative values and the anodic domain current density remains almost the same. This result suggests that the presence of both extracts reduces only the cathodic reaction of hydrogen evolution without affecting anodic dissolution of metal. Since the measurements were carried out in deaerated electrolyte solutions, the cathodic reaction was hydrogen evolution.
The corresponding electrochemical parameteres such as corrosion potential, Ecorr, corrosion current densities, jcorr, cathodic Tafel slope, bc, anodic Tafel slope, ba, along with the inhibition effciency, h are listed in Table 1.
From Table 1 it is evident that by adding inhibitors ba values do not change significantly with respect to the blank solution. However, bc values change significantly so both extracts behave like cathodic inhibitors. An inhibitor can be referred as cathodic or anodic if the displacement in corrosion potential is more than 85 mV with respect to Ecorr of the blank [17, 30]. In our study, the displacement in Ecorr value was less than 85 mV and this indicates that both extracts can be classified as mixed-type inhibitor which is predominantly cathodic.
The inhibition efficiency, h of SM extract and phenolic fraction determined through the corrosion current density is given by the following relation:
Where and jcorr are corrosion current densities of the Fe in the absence and presence of SM or PF. Table 1 shows that the addition of both extracts has better corrosion inhibition effect and increases inhibition efficiency.
3.2 Cyclic voltammetry
The cyclic voltammograms of iron in 0.1 mol L−1 NaCl solution without and in the presence of extract (SM) and phenolic fraction (PF) of Satureja montana L. were recorded and are represented in Figure 2. The potential was started at –1.40 V and swept in the positive direction up to –0.20 V at a scanning rate of 40 mV s-1.
Oxidation and reduction processes on the bare Fe electrode have already been discussed [4,18] and here will be only briefly covered. Two oxidation peaks can be observed in the anodic portion of cyclic voltammogram. The first oxidation peak, A1, at a potential –0.90 V, can be associated to the formation of a non-protective Fe(OH)2 layer. Second peak, A2, in the anodic direction is observed at potential –0.52 V and this is associated with the oxidation of Fe(II) to Fe(III). It is followed by the region with the constant current density up to –0.20 V indicating a steady-state film growth in the passive region. In the reduction scan, two peaks (C2, C1) are observed which are related to the reduction of Fe3+ to Fe2+ and Fe2+ to Fe, respectively. As shown in a previous investigation  the reduction of oxide layer is not complete. The CV spectra (Figure 2) showed that the addition of extract (SM) and phenolic fraction (PF) had no significant effect of the E/j profile in the potential region of peak A1 while in the potential region of peak A2 as well as the reduction processes in the potential region of peaks C2 and C1, were inhibited.
The difference in current passivity plateau was also noticed. The results showed that the addition of both extracts contributed to the iron corrosion inhibition and their inhibitive action could be explained by an insoluble Fe(III)complex that precipitates onto the electrode surface and decreases in the passive current density.
Fe2+ and Fe3+ ions prefer octahedral geometry and can coordinate up to three polyphenol compounds. Polyphenols are so structurally varied and the complexes formed are pH dependent. In near-neutral solutions many polyphenols are easily deprotonated and form stable complexes with iron ions . At oxidation peak, A1, the iron dissolution product, Fe2+ with the polyphenols does not form complex with protective properties [16,32] and in this area the current density does not change with addition of extracts (Figure 2). During anodic polarization at the potential of the second oxidation peak (A2), polyphenols react with Fe3+ ions and form a protective ferric-complex.
This process is facilitated by strong electron donating density of oxygen ligands of the polyphenols that stabilize higher oxidation iron state .
Our analysis of the Satureja montana L. by means of spectrophotometry using Folin-Ciocalteu reagent has revealed the presence of the natural phenolic compounds. The total phenolics were expressed as tannic acid equivalents and it was obtained 194 mg/g of plant material. It is well known that extraction solvents had significant effects on the quantity of phenolic compounds [23,25,33]. The high-performance liquid chromatography (HPLC) [25,34] analysis of composition of the Satureja montana L. has confirmed the presence of the phenolic compounds. Some authors  demonstrated good correlation between phenolic profiles and corrosion inhibition effect.
From the CV measurements, it is possible to determinate the total charges used for both processes (anodic, Q A and cathodic, QC) occurring on the Fe electrode. The QA was estimated by integration of the anodic portion of cyclic voltammograms. Anodic charges determined after addition of Satureja montana L. extract (SM) and phenolic fraction (PF) are used to calculate the inhibition efficiency, h according to the equation:(2)
where and QA represent anodic charges without and with extract (SM or PF). Inhibition efficiency is presented in Table 2 together with anodic charge values.
The calculated inhibition efficiencies obtained from the cyclic voltammetry method are close to the value determined from the potentiodynamic polarization measurements.
3.3 Electrochemical impedance spectroscopy
The mechanism of the electrochemical processes included in the process of oxide film formation and how it is modified by the presence of a corrosion inhibitor has been investigated using impedance spectroscopy. Impedance spectra for Fe electrode without and with SM and PF extract in 0.1 mol L-1 NaCl solution are presented in Figure 3 in the form of the Nyquist and Bode diagrams. The EIS diagrams were obtained at the open circuit potential.
The Nyquist plot obtained in the blank solution revealed one time constant; a single capacitive semicircle, showing that the corrosion process is mainly charge transfer controlled [33,35]. As can be seen, in presence of inhibitor two time constants were observed in the Nyquist diagram: the capacitive loop at high frequencies and a straight line seen as Warburg impedance in the low frequency. The high frequency capacitive loops can be related to the charge transfer process. The second time constant, in the low frequency range, consists of surface layer resistance, R2, surface layer capacity, Q2 and Warburg element that indicates a diffusion process through the surface layer.
The Bode plot obtained in the absence of the inhibitor shows three distinct regions (Figure 3).
In the higher frequency region (f > 1 kHz) the log|Z| values are low and phase angle values fall towards 0°, the data are dominated by electrolyte resistance. In the medium frequency region, the impedance of the system increases linearly with frequency decreases. In this area is appear the maximum phase angle (q = −30°). This value differs considerably from those expected for an ideal capacitor, where q = −90°. At the lowest frequencies the resistive behaviour of the system increases, but the area where |Z| is independent on f is not completely reached.
Bode plots recorded in the presence of inhibitor showed that the Bode amplitude values over the entire frequency range increase with the addition of SM and PF. As further inspection in Figure 3, it can be seen that phase angles increased up to −40° at low frequency area in the presence of the SM and PF indicating a capacitive behaviour. These changes indicate improved corrosion resistance.
Equivalent circuit models shown in Figure 4 are used to simulate obtained impedance data from Figure 3. The standard criteria for evaluation of EEC best-fit were followed: the chi-square error was low (χ2 ≤ 10-4 ) and the acceptable errors of elements in fitting mode (5%). The numerical values of the EEC parameters are presented in Table 3.
A constant phase element (CPE) was used in place of a capacitor to compensate for deviations from ideal dielectric behavior arising from the heterogeneities of the electrode surface . The impedance of CPE is defined by the following equation: where Q is the frequency-independent constant, ω is the angular frequency, j is the imaginary constant and n is the CPE parameter which characterizes the deviation of the system from ideal capacitive behaviour. The value of n is between -1 and 1. For a perfect capacitor n = 1, for a perfect resistor, n = 0, and for an inductor n = -1 .
As can be seen from Table 3, the addition of SM and PF extracts leads to the increase in total resistance, R1 + R2 (from 118 to 332 and 371 W cm2) and decrease in double layer capacity, C dl (from 2004 ·10-6 to 1860·10-6 and 1524 · 10-6 W-1 sn cm-2). The decrease in C dl, which can result from a decrease in local dielectric constant and/or an increase in the thickness of electrical double layer, suggests that the inhibitor molecules adsorbed at the metal/solution interface .
The total impedance of the Fe electrode/0.1 mol L-1 NaCl interface without (3) and with SM and PF extract (4) is given by:(3)(4)
3.4 Atomic absorption spectrometry
Quantitative determination of iron ions concentration in 0.1 mol L-1 NaCl solution obtained by atomic absorption spectrometry are presented in Table 2. Comparing these results, it can be observed that the quantities of released Fe ions are significantly reduced in the presence of both extracts.
It is obvious from Table 4 that the highest quantities of released Fe ions are in the pure NaCl solution (0.498 mg L-1 cm-2). An addition of phenolic compounds (SM and PF) in a NaCl solution reduces the amount of dissolved iron in the corrosive solution, especially in the presence of the PF extract. The results obtained from the AAS method are in accordance with the results recorded by electrochemical techniques. The inhibitive properties may be due to the presence of phenolic compounds in the extracts and formation of ferric complex.
The influence of Satureja montana L. extract and phenolic fraction of Satureja montana L. extract on the electrochemical behaviour of the iron in a 0.1 mol L-1 NaCl was investigated using electrochemical techniques, atomic absorption spectrometry and UV/Vis spectrophotometry.
The presence of the total phenolic compounds in SM extract (194 mg/g; as tannic acid equivalents) was confirmed by UV/Vis spectrophotometry. Results of all techniques showed that the SM and PF contributed to the inhibition process. An inhibition activity is due to the precipitate of Fe-complex layer at the metal surface. According to the EIS results total resistance values were increased by adding SM and PF. The inhibition efficiency for SM extract was around 45% and for PF fraction was about 55%. The concentration of the metallic ions released into solution, measured by atomic absorption spectrometry, was in a good correlation with the results obtained from the electrochemical techniques.
El-Sayed M.S., Erasmus R.M., Comins J.D., In situ Raman spectroscopy and electrochemical techniques for studying corrosion and corrosion inhibition of iron in sodium chloride solutions, Electrochim. Acta, 2010, 55, 3657-3663. Web of ScienceCrossrefGoogle Scholar
Mišković I., Pilić Z., Influence of fluoride concentration and pH value on the corrosion behaviour of iron, Int. J. Electrochem. Sci., 2013, 8, 7926-7937. Google Scholar
Gui J., Devine T.M., A SERS investigation of the passive films formed on iron in mildly alkaline solutions of carbonate/bicarbonate and nitrate, Corros. Sci., 1995, 37, 1177-1189. CrossrefGoogle Scholar
Schmuki P., Büchler M., Virtanen S., Isaacs H.S., Ryan M.P., Böhni H., Passivity of iron in alkaline solutions studied by in situ XANES and a laser reflection technique, J. Electrochem. Soc., 1999, 146, 2097-2102. CrossrefGoogle Scholar
Davenport A.J., Sansone M., High resolution in situ XANES investigation of the nature of the passive film on iron in a pH 8.4 borate buffer, J. Electrochem. Soc., 1995, 142, 725-730. CrossrefGoogle Scholar
El-Sayed M.S., A comparative study on the electrochemical corrosion behavior of iron and X-65 steel in 4.0 wt % sodium chloride solution after different exposure intervals, Molecules, 2014, 19, 9962-9974. Web of ScienceCrossrefPubMedGoogle Scholar
Pilić Z., Natural products as potential corrosion inhibitors, In: S. Tomas, Đ. Ačkar (Ed.), Book of Abstracts of 17th International Conference Ružička days “Today science-tomorrow industry” (19-21 September 2018, Vukovar, Croatia), Faculty of Food Technology Osijek, 2018, 11-11. Google Scholar
Attia E.M., Dipron: an eco-friendly corrosion inhibitor for iron in HCl media in both micro and nano scale particle size - Comparative study, Int. J. Adv. Res., 2016, 4, 986-1003. Google Scholar
Rahim A.A., Rocca E., Steinmetz J., Kassim M.J., Inhibitive action of mangrove tannins and phosphoric acid on pre-rusted steel via electrochemical methods, Corros. Sci., 2008, 50, 1546-1550. Web of ScienceCrossrefGoogle Scholar
Soltani N., Tavakkoli N., Khayatkashani M., Jalali M. R., Mosavizade A., Green approach to corrosion inhibition of 304 stainless steel in hydrochloric acid solution by the extract of Salvia officinalis leaves, Corros. Sci., 2012, 62, 122-135. Web of ScienceCrossrefGoogle Scholar
Pilić Z., Martinović I., Zlatić G., Electrochemical behaviour of iron in simulated acid rain in presence of Achillea millefolium L., Int. J. Electrochem. Sci., 2018, 13, 5151-5163. Web of ScienceGoogle Scholar
Ćetković G.S., Čanadanović-Brunet J.M., Djilas S.M., Tumbas V.T., Markov S.L., Cvetković D.D., Antioxidant potential, lipid peroxidation inhibition and antimicrobial activities of Satureja montana L. subsp. kitaibelii extracts, Int. J. Mol. Sci., 2007, 8, 1013-1027. CrossrefWeb of ScienceGoogle Scholar
Serrano C., Matos O., Teixeira B., Ramos C., Neng N., Nogueira J., Nunesc M.L., Marques A., Antioxidant and antimicrobial activity of Satureja montana L. extracts, J. Sci. Food Agric., 2011, 91, 1554-1560. CrossrefWeb of ScienceGoogle Scholar
Kundaković T., Stanojković T., Kolundzija B., Marković S., Sukilović B., Milenković M., Lakusić B., Cytotoxicity and antimicrobial activity of the essential oil from Satureja montana subsp. pisidica (Lamiceae), Nat. Prod. Commun., 2014, 9, 569-572. PubMedGoogle Scholar
Radonić A., Miloš M., Chemical composition and in vitro evaluation of antioxidant effect of free volatile compounds from Satureja montana L., Free Radical Res., 2003, 37, 673-679. CrossrefGoogle Scholar
Ćavar-Zeljković S., Topčagić A., Požgan F., Stefanec B., Tarkowski P., Maksimović M., Antioxidant activity of natural and modified phenolic extracts from Satureja montana L., Ind. Crops. Prod., 2015, 76, 1094-1099. CrossrefWeb of ScienceGoogle Scholar
Vladić J., Zeković Z., Cvejin A., Adamović D., Vidović S.S., Optimization of Satureja montana extraction process considering phenolic antioxidants and antioxidant activity, Sep. Sci. Technol., 2014, 49, 2066-2072. CrossrefWeb of ScienceGoogle Scholar
Ćetković G.S., Mandić A.I., Čanadanović-Brunet J.M., Djilas S.M., Tumbas V.T., HPLC screening of phenolic compounds in winter savory (Satureja montana L.) extracts, J. Liq. Chromatogr. Related Technol., 2007, 30, 293-306. CrossrefWeb of ScienceGoogle Scholar
Demiray S., Pintado M.E., Castro P.M.L., Evaluation of phenolic profiles and antioxidant activities of Turkish medicinal plants: Tilia argentea, Crataegi folium leaves and Polygonum bistorta roots, World Acad. Sci. Eng. Technol., 2009, 54, 312-317. Google Scholar
Rajabian A., Hassanzadeh Khayyat M., Emami S.A., Tayarani-Najaran Z., Rahimzadeh Oskooie R., Asili J., Phytochemical evaluation and antioxidant activity of essential oil, and aqueous and organic extracts of Artemisia dracunculus, Jundishapur J. Nat. Pharm. Prod., 2017, 12(1), e32325, DOI: 10.5812/jjnpp.32325. Web of ScienceGoogle Scholar
Kadapparambil S., Yadav K., Ramachandran M., Selvam N.V., Electrochemical investigation of the corrosion inhibition mechanism of Tectona grandis leaf extract for SS304 stainless steel in hydrochloric acid, Corros. Rev. 2017, 35, 111-121. Web of ScienceGoogle Scholar
Perron N.R., Brumaghim J.L., A review of the antioxidant mechanisms of polyphenol compounds related to iron binding, Cell Biochem. Biophys., 2009, 53, 75-100. Web of SciencePubMedCrossrefGoogle Scholar
Tan K.W., Kassim M.J., A correlation study on the phenolic profiles and corrosion inhibition properties of mangrove tannins (Rhizophora apiculata) as affected by extraction solvents, Corros. Sci., 2011, 53, 569-574. CrossrefWeb of ScienceGoogle Scholar
López-Cobo A., Gómez-Caravaca A.M., Švarc-Gajić J., Segura-Carretero A., Fernández-Gutiérrez A., Determination of phenolic compounds and antioxidant activity of a Mediterranean plant: The case of Satureja montana subsp. kitaibelii, J. Funct. Foods, 2015, 18, 1167-1178. CrossrefWeb of ScienceGoogle Scholar
Ostovari A., Hoseinieh S.M., Peikari M., Shadizadeh S.R., Hashemi S.J., Corrosion inhibition of mild steel in 1M HCl solution by henna extract: A comparative study of the inhibition by henna and its constituents (Lawsone, Gallic acid, α-d-Glucose and Tannic acid), Corros. Sci., 2009, 51, 1935-1949. CrossrefWeb of ScienceGoogle Scholar
Metikoš-Huković M., Pilić Z., Babić R., Omanović D., Influence of alloying elements on the corrosion stability of CoCrMo implant alloy in Hank’s solution, Acta Biomater., 2006, 2, 693-700. CrossrefPubMedGoogle Scholar
Orazem M.E., Tribollet B., Electrochemical Impedance Spectroscopy, Willey, 2008. Google Scholar
About the article
Published Online: 2019-12-10
Conflict of interest: Authors declare no conflict of interest.